1. Field of the Invention
The present invention relates to an image processing apparatus and method for processing an image by mean of digital signals and, more particularly, to an image processing apparatus and method for quantization processing concentration data of an image.
2. Related Background Art
What are called digital copying apparatuses in which, in general, an image is sampled by a CCD sensor or the like, the digitized data is output from a digital printer such as a laser beam printer or the like and the image is reproduced, are being widely used in place of the conventional analog copying apparatuses.
Such digital copying apparatuses generally use a system in which gradations are reproduced by a dither method or concentration pattern method to reproduce a half-tone. However, these methods have a following problems.
(1) In the case where an original is a mesh point image such as a print or the like, a periodic fringe pattern which does not exist on the original appears in the copied image.
(2) When an original includes graphic images, characters, and the like, their edges are reproduced like stairway images as a result of the dither process, so that the image quality deteriorates.
The above phenomenon, item (1) is called a moire phenomenon and the following points are considered as the causes for generation of such a phenomenon.
(A) Beats between frequencies of the mesh point original and the input sampling.
(B) Beats between frequencies of the mesh point original and the dither threshold value matrix.
Particularly, according to the phenomenon which is generated by the cause of (B), in general, when the threshold values of the dither are arranged as a dot concentration type, an output image also has a pseudo-mesh-point structure. Beats are generated by this pseudo-mesh-point structure and input mesh point original, so that the moire phenomenon occurs.
On the other hand, an error diffusion method or a least mean error method which is substantially the same as the error diffusion method is known as another binarizing method. According to this method, the concentration difference between the image concentration of an original and the output image concentration is arithmetically calculated for every pixel and the error amount so calculated is dispersed after special weights are added to the peripheral pixels. This method has been published in the reference by R. W. Floyd and L. Steinberg, "An Adaptive Algorithm for Spatial Gray Scale", SID 75 Digest, pages 36 and 37, 1975.
According to this method, since there is no periodicily, unlike the dither method, no moire is generated for the mesh point image. However, there are drawbacks, such that a special fringe pattern appears in the output image particle-like noises are conspicuous in the highlight and dark portions of the image, and the like.
To prevent the occurrence of such fringe pattern and particle-like noises, the region to disperse the error between the original image concentration and the output image concentration is changed in accordance with an edge quantum or an image concentration. This method has been disclosed in our commonly assigned U.S. application Ser. No. 07/137,439, filed Dec. 23, 1987.